However, under new proposals by Gordon Brown, in future, everything in the UK will be illegal until you can prove it's not. In the interests of justice, though, you will be given 42 days in which to defend yourself - but only if you can guess what the charge is...
Yes, the operative word in this sentence being "off" the engine has to be switched off for you to be on the right side of the law, so no problem there. What if the car you are in is electric, or solar powered, though?